The present invention is related to an improved method of forming a solid electrolyte capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to an improved cathode where the cathode comprises highly conductive polymer dispersion coatings with enhanced reliability. The invention also discloses methods for manufacturing the intrinsically conductive polymer dispersions. The present invention is related to an improved method of forming a solid electrolyte capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to a capacitor with improved leakage stability.
The construction and manufacture of solid electrolyte capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover all surfaces of the anode and serves as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide or electrically conductive organic materials such as polyaniline, polypyrrole, polyethylenedioxythiophene and their derivatives. Solid electrolytic capacitors with intrinsically conductive polymers as the cathode material have been widely used in the electronics industry due to their advantageous low equivalent series resistance (ESR) and “non-burning/non-ignition” failure mode. In the case of conductive polymer cathodes the conductive polymer is typically applied by either chemical oxidation polymerization, electrochemical oxidation polymerization or spray techniques with other less desirable techniques being reported.
The backbone of a conductive polymer consists of a conjugated bonding structure. The polymer can exist in two general states, an undoped, non-conductive state, and a doped, conductive state. In the doped state, the polymer is conductive but of poor processability due to a high degree of conjugation along the polymer chain, while in its undoped form, the same polymer loses its conductivity but can be processed more easily because it is more soluble. When doped, the polymer incorporates anionic moieties as constituents on its positively charged backbone. In order to achieve high conductivity, the conductive polymers used in the capacitor must be in doped form after the completion of the process, although during the process, the polymer can be undoped/doped to achieve certain process advantages.
Various types of conductive polymers including polypyrrole, polyaniline, and polyethyldioxythiophene are applied to the Ta capacitors. The major drawback of conductive polymer capacitors, regardless of the types of conductive polymers employed, is their relatively low working voltage compared to their MnO2 counterparts. The polymer capacitors have reliability issues, to varying degrees, when the voltage rating exceeds 25V. This is believed to be caused by the relatively poor dielectric-polymer interface, which has poor “self-healing” capability. The ability to withstand high voltage can be best characterized by the breakdown voltage (BDV) of the capacitors. Higher BDV corresponds with better reliability. For reasons which were previously unknown the break down voltage of capacitors comprising conductive polymers has been limited to about 55V thereby leading to a capacitor which can only be rated for use at about 25V. This limitation has thwarted efforts to use conductive polymers more extensively.
U.S. Pat. No. 7,563,290, which is incorporated herein by reference, describes the slurry/dispersion process. The resulted capacitors show excellent high voltage performances, reduced DC leakage (DCL) and improved long term reliability.
It is highly desirable that the capacitor devices are of high reliability and that they can withstand stressful environments. Therefore, the integrity of the anodes and the robustness of conductive polymer cathode are essential for high quality capacitor products. However, it is a challenge to form a conductive polymer coating on the anodes that is defect-free, and which is capable of withstanding thermal mechanical stress during anode resin encapsulation and surface-mounting. The improper application of polymer slurry often leads to the formation of cracks and delaminating of the polymer coating thus formed.
In a manufacturing process to produce conductive polymer based valve metal capacitors the valve metal powder, such as tantalum, is mechanically pressed into anodes that are subsequently sintered to form porous bodies. The anodes are anodized to a pre-determined voltage in a liquid electrolyte to form a dielectric layer onto which a cathode layer of conductive polymer is subsequently formed via an in situ polymerization process comprising multi-cycle oxidizer/monomer coatings and polymerization reactions. The anodes are then coated with graphite and Ag followed by assembling and molding into a finished device.
Today, almost all electronic components are mounted to the surface of circuit boards by means of infra-red (IR) or convection heating of both the board and the components to temperatures sufficient to reflow the solder paste applied between copper pads on the circuit board and the solderable terminations of the surface mount technology (SMT) components. A consequence of surface-mount technology is that each SMT component on the circuit board is exposed to soldering temperatures that commonly dwell above 180° C. for close to a minute, typically exceeding 230° C., and often peaking above 250° C. If the materials used in the construction of capacitors are vulnerable to such high temperatures, it is not unusual to see significant shifts in ESR and leakage which lead to negative shifts in circuit performance.
The state of the art for inherently conductive polymer (ICP) dispersions has a number of issues when solid electrolytic capacitors with conductive polymer dispersions are exposed to SMT conditions.
The presence of moisture in the materials used in capacitors can cause poor package integrity due to SMT reflow conditions. Conductive polymer dispersions have relatively high moisture sorption in comparison with insitu polymerized conductive polymer. Presence of hydrophilic polyanions, specifically polystyrene sulfonic acid, is one of the reason for the high moisture sorption. Insitu polymerized parts use monomeric dopant in comparison to polymeric dopants, polyanions, used in dispersions. When heated to a temperature higher than the boiling point of water, which occurs during mounting of the capacitor on the mounting substrate by solder reflow, moisture contained in the capacitor element of the capacitor is vaporized which increases the internal pressure of the mold resin. Since the capacitor is rapidly heated to the solder reflow temperature, which is substantially higher than the boiling point of water, the internal pressure of the capacitor is increased substantially and rapidly. In such cases, since the capacitor element is completely encapsulated by the humidity resistant mold resin of such as epoxy resin, vapor thus generated in the capacitor cannot escape through the mold resin, so that all high pressure due to water vapor is exerted on the mold resin. As a result, portions of the mold resin are cracked and water vapor in the mold resin release through the cracks. This is particularly a problem in thin portions of the resin such as near connection portions and on the lower surface side.
U.S. Pat. No. 6,229,688 discloses a method to reduce case integrity/case cracking by providing a water release mechanism. The solid electrolytic capacitor features a water vapor passage formed in the mold resin. The water vapor discharge passage is formed of a material having higher water vapor permeability than that of the mold resin and functions to communicate an interior of the mold resin to atmosphere.
A number of approaches are reported to improve moisture resistance of the materials in the capacitor. One approach to improving moisture resistance is provided in U.S. Publ. Pat. Appl. No. 20100254072 wherein conductive polymer dispersions and solid electrolytic capacitors are taught to have a low ESR and an excellent moisture resistance due to incorporation of a sulfonic acid ester compound in the conducting polymer.
Another approach is provided in U.S. Publ. Pat. Appl. No. 2011/0019340 where the electrically conductive polymer suspension comprises dopant composed of a polyacid or a salt thereof; at least one compound selected from erythritol, xylitol and pentaerythritol; and a dispersion medium. U.S. Publ. Pat. Appl. No. 20060223976 provides a conductive polymer excellent in conductivity, heat resistance and moisture resistance, by including at least one organic sulfonate having an anion of an organic sulfonic acid, that is the same or different from the organic sulfonic acid incorporated in the conductive polymer as a dopant, and a cation other than transition metals.
Another approach is disclosed in U.S. Publ. Pat. Appl. No. 2010/0091432 wherein the conductive polymer includes a conductive polymer, a polyanion that includes a hydrophilic group, where the polyanion functions as a dopant of the conductive polymer. Further, at least a part of the hydrophilic group of the polyanion is condensed with an epoxy group in a compound with one epoxy group.
Another problem with conductive polymer containing solid electrolytic capacitor is the high leakage at high temperatures and after surface mount conditions. It is theorized that one of the causes of this high leakage under these conditions is the lack of sufficient moisture in the ICP coating. A lack of moisture in the ICP material during processes, such as surface mount, can cause leakage in solid electrolytic capacitors.
U.S. Pat. No. 7,773,366 discloses a method for incorporating a water retaining layer to improve leakage and other electrical characteristics of a solid electrolytic capacitor. In this method, a water-retaining layer having higher water absorption than that of the housing is placed between the conductive polymer layer and the housing. The water absorption of the housing is preferably 0.04% or less. Thereby, water dissipated to the outside through the housing can be suppressed, and the reduction in the content of internal water can be prevented. As the water-retaining layer, an epoxy resin can be used, and the water-retaining layer can be formed by applying a liquid epoxy resin.
U.S. Publ. Pat. Appl. No. 2006/0240593 discloses a method for improving leakage current by incorporating organic compounds having a boiling point of not lower than 150° C. and a melting point of no higher than 150° C. While some of the above mentioned references claim moisture resistance improvement, other reference claim improved performance with a water retaining layer. However these approaches do not address the need for a balance between moisture resistance/low moisture sorption and moisture retention for solid electrolytic capacitor with improved reliability. The above approaches also do not address issues related with SMT reflow exposures.
Thus, there is a need for a process for forming solid electrolytic capacitors with improved leakage and leakage stability. A particular need is for capacitor parts to have stable leakage during surface mount temperatures.
Thus, a need exists for the proper management of moisture which is required to produce solid electrolytic capacitors with excellent reliability. Here to fore it has not been recognized that a delicate balance of moisture content and moisture retention properties are required to simultaneously avoid poor package integrity and high leakage current after being subjected to SMT conditions.